Mark,
I think this is the issue I had with my 261 and its 848 head. Blocked holes in the head with the gasket...

https://www.stovebolt.com/ubbthread...in=162425&Number=1102016#Post1102016

Maybe this will help.
Fox


1970 Chevrolet C10
Grandpa's -- My first truck -- In progress to shiny
Follow the build in the Project Journal
1950 Chevrolet 1-Ton Dually
"Ole Red Girl"
In the Stovebolt Gallery
More pictures here
1951 GMC 9430 1 ton dually--Shiny! | 1972 Chevrolet C20- Rusty- the puzzle box lid for the C10 | 1962 AMC Rambler American- my wife's
Parts trucks-
1951 GMC 9300 | 1951-GMC 9430 | 1951- Chevrolet 1300